Scavenging of radioactive aerosols



United States Patent 0,

3 113 007 SCAVENGHNG or inactive AEROSOLS Jan Rosinski, Cicero, andDonald K. Werle, Franklin Park, 11]., assignors to the United States ofAmerica as 3 1 13 ,007 Patented Dec. 3, 1963 silicon tetrafluoride. Theneutralization agents best suitable were sodium metasilicate and sodiumhydroxide; they are preferably added in the form of a dilute aqueoussolution, so that a separate addition of water is not neces- 5 sary. Theconcentration of these solutions may range rmegrsigtiezgnted by theUnited States Atomic Energy Com from 1 to 2 M. Instead of mixing theneutralization N0 Drawing Filed May 11 1961, Se 109,467 agent with thesilicon tetrafiuoride, it can be introduced 6 Claims. 1. 55 also shortlyafter the silicon tetrafiuoride has been added. In order to study theefiect of various substances on This invention deal with thprecipitation of radio c- 10 the behavior of aerosols, a radioactiveatmosphere was tive suspended material from the atmosphere. Theprobartificially createdthis P p a g Wife was blem of decontaminatingatmospheres from radiocative e ron-bombarded and mounted in an about21-inch aerosols arises after a nuclear accident in a reactor, and it gLuclte chamber, inqlde Wa f W ch had been also becomes important fortreating radioactive atmos- Coated With a Cohdllchhg ant-18mm P The Wirepheres released by nuclear-powered ships. was mounted between goldelectrodes, and the chamber It is an object of this invention to providea process by Was Closed and g y evacuated to a t 28 inches of whichradioactive aerosols of the type described above Hg absolute pressure.An electric current of 6000 volt can be precipitated in a comparativelyfast and quantita- Was then dlschafged gh the Wlfe whereby the 13am tivemanne exploded and disintegrated into minute particles. After It is afurther object of this invention to provide a 20 t eXP10S10h, filteredair Was admitted to Chamber; process for the precipitation ofradioactive aerosols which It took from 2 t0 3 mmhths with thePressfllre had reachfid does not require a li d apparatus, oneatmosphere. After this the chamber [was closed A great number of gaseousand liquid precipitating g I agents where investigated for the removalof radioactive The Chamber h a WI-IIdO'W 6 1IiC eS above thebotaerosols, but found unsatisfactory; silicon tetraflu-oride, throughWhlch the vltv ld be vlewed however, proved to be well qualified. It wasfound that, and measllhed y h t a Solnhflahon Counter an While siliconfluoride improves the precipitation of the o e WIth a scmhllatwhspectrometef- Of Course, aerosols, a mixture of it with water is stillmore effective. as th aero ol ag la e an set led to the bottom, the Aexplanation f th outstanding suitability f ili rad1oaot1v1ty in theupper space of the chamber decreased. .tetrafluoride for the purpose ofthis invention cannot be he background actlvlty was also measured 111each 0356 given, and deducted from the readings in the chamber.

The process of this invention thus comprises introduc- The fhllowihg pgives a oompalhisoh of Various ing silicon tetrafluoride with or withoutwater into the scavellglhg gasesatmosphere to be treated and removingthe precipitated substance from the atmosphere. EXAMPLE I The quantityof siilicon tetrafluoride may vary widely; A radioactive gold aerosolwas prepared in each run amounts as low as 0.044% by volume have beenfound to as described above. The sedimentation rates were meashave apronounced beneficial effect on the settling time ured from time to timeby determining the radioactivity of the radioactive aerosol. The mostadvantageous quanat the level of the Window of the chamber; thesemeasuretity of water is that between 1.5 and 3% by volume as ments wereused for calculating the time when the radioreferred to the amount ofsilicon tetrafluoride added. activity had reached a value of 90, 80, 70,60, 50, 40, 30, Because silicon tetrafluoride has a tendency to hy- 20and 10%, respectively. The conditions used and the drolyze in thepresence of Water and because the hyresults of the calculations from themeasurements, exdrolyzation products have corrosive and toxicproperties, pressed in minutes after the exposion, are compiled in itwas found advisable to add a neutralizing agent to the Table I.

Table I Erpt. No I II III IV v VI VII VIII IX X XI XII Irradiation, mc 11 1 1 1 1 1 16 1 1 1 1 R01. humidity, percent 31 00-100. 40 32 00-100-100 24 00-100 00-100 00-100 00-100 Temp, F 70 73 75 8O 07 00 77 81 7882 Approx. density,

g./cc -10.3 -1.o -1.0 -1.0 -1.5 -1.0 -1.0 -10.3 -10.3 -10.3 -1.0 -10Scavenger used, v/o. None 0.153 0.153 0.153 SlFl 0.153 NH: 0044 0.153NH; None Fine Coarse Fine Coarse (0111020, 81F; 0.153 NH; 0.153 H01 SiFAexcess water water Water water 0.153 B HOOOH spray spray spray spray,

vapor 0.044 @044 SlF 51F.

Radio-activity, percent:

It will be seen from Table I that the gold used for all experiments hadbeen irradiated with 1 me, with the exception of experiment VHEI wherethe gold had been exposed to 16 me. The least efiective scavengers werethose used without water and those of a non-hygroscopic nature, such asammonium chloride (experiment V) and boron trichloride-ethylene oxidemixture (experiment I'I). Hygroscopic scavenging agents were found to besuperior in all cases. By far the best results were obtained withsilicon tetrafiuoride used in combination with a fine water spray, lessthan 14 minutes being required for a 90% precipitation.

Because hydrolysis products of silicon fluoride proved to be corrosive,replacement of this scavenging agent by the hygroscopic ammonium formatewas investigated. However, ammonium formate proved only about half aselfective as silicon tetrafluoride of the same concentration, as will beobvious from a comparison of experiments VII and Ill.

The effect of degree of radiation exposure was also determined incarrying out experiment VIII with a gold aerosol having received 16 me.and comparing the results of this experiment with those of experiment I.In both instances no scavenging agent was used. The gold that receivedthe higher irradiation level settled fast-er.

Because water vapor in all instances showed such a beneficial effect onthe precipitation, the effect of water particles alone was also tested(experiments IX and X). However, the results of these experiments werenot satisfactory and by far not as good as those obtained with thesilicon tetrafiuoride-water combinations. This clearly shows that thesilicon tetrafluoride, and not the water, is responsible for the goodscavenging effects obtained in experiments XI and XI I.

Silicon tetrachloride was also investigated as a scavenging agent,because it does not yield toxic or as strongly corrosive hydrolysisproducts as does the silicon tetrafluoride. 'It was found to beoperative but yielded considerably less satisfactory results than thesilicon tetrafluoride. This again shows the outstanding place silicontetrafluoride occupies as a scavenger for the purpose of this invention.

EXAMPLE II 'I wo parallel experiments were carried out, experiment Iusing 0.044 vol. percent of silicon tetrafiuoride at room temperaturetogether with 1.04 N sodium hydroxide at a relative humidity of between90 and 100%, experiment II using the same conditions but only a relativehumidity of 23%. These two experiments were compared with experiment XIof Example I in which a mixture of silicon tetrafiuoride and water(relative humidity from 90 to 100%), without sodium hydroxide, was usedas the scavenger. Also in these instances a gold wire was electricallyexploded as in Example I, and the chamber was evacuated to 28 inches ofHg prior to the introduction of silicon tetrafluoride. The results ofthese experiments are compiled in Table II.

It will the obvious from the data below that the presence of sodiumhydroxide did not have any beneficial eifect on the scavengingelficiency. However, in the instance where the humidity was between 90and 100% (experiment I), the precipitation results compared favorablyTable II Expt. I Expt. II Expt. XI of Example I Time, min. Time, min.Time, min. Radioactivity, Percent Sodium Sodium Distilled Hydroxide,Hydroxide, Water, 0.044% sum, 0.044% SiFa, 0.044% SiF 68 F., SID-100% 68F., 23% 78 F., 9 100% RH. RH. RH.

with those of experiment Xi where no sodium hydroxide was used, this inparticular in the first half of the precipitation procedure (up to a 50%precipitation). When the humidity was only 23%, it took three times aslong to reduce the radioactivity by 50% as when the humidity was aboveThus it proved to be advantageous to use a high humidity with thesilicon tetrafluoride and sodium hydroxide to avoid harmful elfects ofhydrolysis products.

it will be understood that this invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

1. A process of decontaminating an atmosphere from radioactive particlessuspended therein, comprising introducing silicon tetrafluoride intosaid atmosphere whereby said particles precipitate, and separating saidparticles from said atmosphere.

2. The process of claim 1 wherein water vapor is added to the atmosphereso that precipitation occurs in the presence of said water vapor.

3. The process of claim 1 wherein water vapor is added in a quantity offrom 1.5 to 3 percent by volume Of S1134- 4. The process of claim 1wherein a neutralizing agent is added to the atmosphere'to'be treatedtogether with the silicon tetrafluoride.

5. The process of claim 4 wherein the neutralizing agent is a sodiumhydroxide solution of a concentration of from 1 to 2 M.

6. The process of claim 4 wherein the neutralizing agent is a sodiummetasilicate solution of a concentration of from 1 to 2 M.

References Cited in the file of this patent Nuclear Science Abstracts:scavenging of Particulate Matters and Aersols in Connection withNuclear-Powered Ships, vol. 15, No. 11, p. 1830, John Rosinski,published November 1960.

Nuclear Science Abstracts: scavenging of Particulate Matter inConnection with Nuclear-Powered Ships, J. Rosinski, vol. 15, No. 11, p.1979, published July 29, 1960.

Nuclear Science Abstracts: scavenging of Aersols in Connection withNuclear-Powered Ships, John Rosinski, vol. 15, No. 11, p. 1979;published Feb. 17, 1961.

1. A PROCESS OF DECONTAMINATING AN ATMOSPHERE FROM RADIOACTIVE PARTICLESSUSPENDED THEREIN, COMPRISING INTRODUCING SILICON TETRAFLUORIDE INTOSAID ATMOSPHERE WHEREBY SAID PARTICLES PRECIPITATE, AND SEPARATING SAIDPARTICLES FROM SAID ATMOSPHERE.